Distortion may be considered to be an alteration of the original shape (or other characteristic) of an object, image, sound, waveform or other form of information or representation. Distortion is usually unwanted. In some fields, distortion is desirable, such as electric guitar (where distortion is often induced purposely with the amplifier or an electronic effect to achieve an aggressive sound where desired). The slight distortion of analog tapes and vacuum tubes is considered pleasing in certain situations. The addition of noise or other extraneous signals (i.e., hum, interference) is not considered to be distortion, though the effects of distortion are sometimes considered noise.
In telecommunication and signal processing, a noise-free “system” can be characterized by a transfer function, such that the output y(t) can be written as a function of the input x asy(t)=F(x(t))When the transfer function comprises only a perfect gain constant A and perfect delay T,y(t)=A·x(t−T)the output is undistorted. Distortion occurs when the transfer function F is more complicated than this. If F is a linear function, for instance a filter whose gain and/or delay varies with frequency, the signal may experience linear distortion. Linear distortion will not change the shape of a single sinuosoid, but will usually change the shape of a multi-tone signal.
A high-pass filter will distort the shape of a square wave by reducing its low frequency components. This is seen as “droop” on the top of pulses. This “pulse distortion” can be very significant when a train of pulses must pass through an AC-coupled (i.e., high-pass filtered) amplifier, for example. As the sine wave contains only one frequency, its shape is unaltered. A low-pass filter will round the pulses by removing the high frequency components. All systems may be considered low pass to some extent. Note that the phase of the sine wave is different for the lowpass and the highpass cases, due to the phase distortion of the filters.
A slightly non-linear transfer function, such as a gently compressing function of a tube audio amplifier, will compress the peaks of the sine wave. This will cause small amounts of low order harmonics to be generated. A hard-clipping transfer function will generate high order harmonics. Parts of the transfer function are flat, which indicates that all information about the input signal has been lost in this region.
The purpose of a power amplifier (PA) is to boost a radio signal to a sufficient power level for transmission through the air interface from the transmitter to the receiver. One goal in RF amplification is to achieve good linearity with reasonable efficiency by applying linearization techniques. An important goal in linearization is to apply external linearization to a reasonably efficient but non-linear PA so that the combination of the linearizer and PA satisfy the linearity specification. Linearization can be thought of as a cancellation of distortion components. There are several sources of distortion and the distortion generated in any of these sources can undergo subsequent mixing processes, resulting in higher order distortion components than the degree of the nonlinearity suggests.
Distortion may originally be generated in the form of current, which is converted to a voltage by terminal impedance. Thus the phase and amplitude of the distortion components can be strongly influenced by the terminal impedances, and especially by the impedances of the biasing networks. One distortion analysis technique is Volterra analysis, which is based on placing polynomial distortion sources in parallel with linear transfer functions. Benefits of this technique include the ease of identification of dominant sources of distortion; identification of phase relationships between distortion contributions; accuracy of a polynomial model for fitting to measured data; and the implementation of polynomial models in harmonic balance simulators.